b43: mark some vars __maybe_unused in b43_nphy_pmu_spur_avoid

Avoids this warning when CONFIG_B43_BCMA is not set:

  CC [M]  drivers/net/wireless/b43/phy_n.o
drivers/net/wireless/b43/phy_n.c: In function ‘b43_nphy_pmu_spur_avoid’:
drivers/net/wireless/b43/phy_n.c:4052:6: warning: unused variable ‘pmu_ctl’
drivers/net/wireless/b43/phy_n.c:4051:22: warning: unused variable ‘cc’

Signed-off-by: John W. Linville <linville@tuxdriver.com>